Inductive sensor for target parameter detection and magnetic image feature determination

ABSTRACT

Inductive sensor for non-contact detection of discontinuities of a conductive or ferromagnetic target for determination of target position, movement and speed, or generally of a magnetic image, including a plurality of receiving secondary windings associated with each exciting primary winding, configured to obtain electric signals optimized as a function of the discontinuities of the target. The sensor, combined with a suitable electric measuring circuit, allows global analysis of the signals provided by the secondary windings and configuration of a network in accordance with a predefined selection. An adaptive circuit allows auto-calibration of the sensor.

FIELD OF THE INVENTION

The invention relates to inductive sensors of planar type, which allowdetection of the discontinuities of a conductive or ferromagnetic part(or target) without contact therewith.

BACKGROUND OF THE INVENTION

Magnetic induction sensors of planar type are known and are described inEuropean Patent Applications No. 97 106 884.6 and No. 98 400 835.9 forexample. Such a miniaturized inductive sensor comprises at least oneprimary winding traversed by an alternating current which creates analternating magnetic field and at least one secondary winding subject tothis alternating field, the windings being planar. The amplitude and thephase of the magnetic flux passing through each secondary winding aremodified by the passage of a metallic or ferromagnetic part (or target)having at least one discontinuity, such that the passage of thisdiscontinuity is detected by measuring the amplitude or phase modulationof the electric voltages induced in the secondary windings, which allowsdetermination of the type of discontinuity and the sense and speed ofthe movement of the metallic part through suitable processing of thesignals representing these voltages.

Such sensors are used in many fields and, particularly in the field ofautomobiles, to detect the speed of rotation of the engine shaft or ofwheels, by means of a toothed wheel for example, whose teeth pass infront of the inductive sensor, which creates variations in the voltageinduced in the secondary windings during the passage of the flanks ofthe teeth. The amplitude of the variations depends strongly on theposition and orientation of the sensor relative to the target. Thus thegeometry of the coils and the mounting of the sensor are adapted to theshape of the teeth, or marks, of the target.

In most applications, the inductive sensor has to be located withprecision relative to the target, so as to obtain a satisfactoryelectric signal, for example with as high as possible a signal-to-noiseratio, to ensure good detection even in the presence of noise introducedby the elements used to process the signal. Such precise positioning ofthe sensor relative to the target is difficult to effect however, orholding its position with the passage of time cannot always be ensuredand the initial adjustment then gets modified.

One object of the present invention is thus to provide an inductivesensor which allows the signal-to-noise ratio to be optimized, even inthe event of non-optimal positioning of the sensor relative to thetarget.

Another object of the present invention is also to provide an inductivesensor whose positioning relative to the target is simpler, cheaper,more rapid, more reliable and thus less troublesome.

A further object of the invention is to allow the use of a large numberof receiving coils whose output signals are processed in an overallmanner in such a way as to allow information to be recovered, forexample on the shape of the discontinuity of the target (magneticimage), or the position or the movement of a predetermined shape.

SUMMARY OF THE INVENTION

The invention thus provides an optimized inductive sensor of the typecomprising a primary winding fed with an alternating current supplied byan excitation device and multiple secondary windings subject co themagnetic field created by the primary winding and providing electricsignals at the output terminals representative of the variations in themagnetic field due to the presence of a metallic target withdiscontinuities, the said electric signals being applied to anamplification-demodulation circuit followed by a processing circuit forthe demodulated signals, characterized in that:

the secondary windings have minimum dimensions compatible with detectionof the signals induced by the variations in the magnetic field, in sucha manner as to be able to associate a maximum number of secondarywinding with a primary winding between two discontinuities of thetarget, and

the output terminals of the secondary windings are connected tointerconnection means which effect adaptive connection of the saidoutput terminals in such a manner as to connect the secondary windingsas a function of the discontinuities of the target, in a stored form oras a desired transfer function.

The optimized sensor of the invention can advantageously be associatedwith an electronic interface circuit through which one or more of thefollowing functions can be executed:

local or global analysis or analysis according to given groupings of thesignals provided by the secondary windings;

grouping of the signals provided by the secondary windings according toa defined representation;

grouping of the signals provided by the secondary windings according toa configuration determined by the interface circuit itself to apply aspecified transfer function.

In the case of adaptation to the marks of the target, couples ofsecondary windings are grouped by the said interconnection means toobtain signals whose signal-to-noise ratio is as good as possible.

The interconnection means can be realized by metallic conductorelements, by electronic switches whose opening and closing are effectedthrough electric signals provided by an adaptive circuit. Theinterconnections can also be determined by an auto-calibrationprocedure. They can for example be realized by means of predefined metalconnections or miniature electronic switches, whose closing depends onthe desired regrouping, or on the local or global transfer functionwhich it is desired to implement. Moreover, it is possible to add acalculation and/or weighting function to all or a part of the secondarywindings.

BRIEF DESCRIPTION OF THE DRAWINGS

Other features and advantages of the present invention will appear froma reading of the following description of one particular embodiment, thesaid description being given with reference to the accompanyingdrawings, in which:

FIG. 1 is a functional diagram of an inductive sensor with the featuresof the present invention,

FIG. 2 is a diagram showing one particular arrangement of a plurality ofsecondary windings relative to the teeth of a wheel,

FIG. 3 is a diagram showing a wiring of the various secondary windingsof the sensor of FIG. 2 in the case of adapting to a target of knownpitch, and

FIG. 4 is a diagram showing an embodiment of auto-calibration of aninductive sensor to the pitch of a target.

BRIEF DESCRIPTION OF THE PREFERRED EMBODIMENTS

An inductive sensor according to the invention is shown in FIG. 1. Itcomprises two parts; the one, referenced 10, comprises the primary andsecondary windings 14 and 16 respectively and the other, referenced 12,is formed by a processing system for the electric signals applied to theprimary winding 14 and those provided by the secondary windings 16.

According to the invention, a plurality of receiving secondary windings16 are associated with respective exciting primary windings 14. Theinput terminals of the primary winding 14 and the output terminals ofthe secondary windings 16 are connected to the processing system 12 byconductors 18. The processing system 12 comprises an electronicexcitation device 20 which provides the electric excitation signal forthe primary winding or exciting coil 14 and an electronic device 22 forprocessing the electric signals appearing at the output terminals of thesecondary windings 16.

The electronic excitation device 20 essentially comprises an oscillator24, whose alternating signal is amplified in an amplifier 26 beforebeing applied to the input terminals of the primary winding 14.

The electronic processing arrangement 22 comprises, according to theinvention:

an interconnection circuit 28 for the output terminals of the secondarywindings 16 such as to group the receiving coils according to apredefined representation, a desired transfer function or in such amanner as to ensure the best possible optimum grouping, where the lattercan be determined by auto-calibration means.

a local processing circuit 28 for the signals provided by the receivingcoils, for effecting a calculation or a given transfer function,

an amplification-demodulation circuit 32 comprising oneamplifier-demodulator per grouping of secondary windings for example,

an adaptive circuit 36 for adapting the interconnection 28 as a functionof the results of the processing effected, and

a processing circuit 34 for the demodulated signals.

The interconnection circuit 28 which effects the optimum grouping of thesecondary winding to obtain a selected representation can be realizeddirectly on the substrate which supports the secondary windings, as hasbeen described in the patent applications referenced above. In thegeneral case, such as is shown in FIG. 1, the interconnection circuit 28allows connection in series of any grouping of receiving windings insuch a manner that this grouping corresponds to the desiredconfiguration. This configuration can be fixed and the interconnectioncircuit 28 is then formed by simple connections, resulting in the seriesconnection of the receiving coils pertaining to the said grouping. Itcan equally be capable of modification and, in this case, theinterconnection circuit advantageously has the form of a matrix of rowsand columns with interconnection means between each row and each column.Each row is moreover connected to first terminal of a receiving windingand each column is connected to a second terminal of a receivingwinding. The interconnection means can be of any known type, such aselectronic mini-switches, fusible links, etc. They should however enablerealization of any form of grouping of the receiving coils to obtain thedesired function. The establishment of a connection between a row and acolumn of the matrix is controlled by the adaptive circuit 36, whosepurpose and function will be explained in the course of the description.

The interconnection circuit 28 has been located ahead of theamplification-demodulation circuit 32 but it can equally be locatedafter the amplification-demodulation circuit 32.

The processing circuit 34 can be of any known type, for example the typeused to process optical images, often called an “artificial retina”, fordetecting the contour, the position, the movement, the orientation andthe speed of objects. The circuit 34 can for example extract the contourof a mark (or tooth) of the target and adapt the configuration, throughthe circuit 28, of the receiving coils, in such a manner that itcorresponds to this contour. Reference may be made in particular to thearticle by P. Vernier et al with the title “An integrated cortical layerfor orientation enhancement” appearing in IEEE Journal of Solid-StateCircuits, Vol. 32, No. 2, Feb. 1997.

In order to effect the operation of demodulation in theamplification-demodulation circuit 32, this receives the electric signalfrom the oscillator 24 of the excitation device 20, which serves as thereference signal for the demodulators.

FIG. 2 is a diagram which shows the positions of the secondary windingof an inductive sensor relative to the vertical teeth of a target, whileFIG. 3 is a diagram which shows an embodiment of a wiring circuitbetween the couples of secondary windings such as is defined by thediagram of FIG. 2.

FIGS. 2 and 3 show an example of the interconnection of the receivingcoils meeting a predetermined configuration. The exciting primarywinding 14 is associated with twelve receiving secondary windings 16,referenced ES1 to ES12 in FIG. 3 and the whole unit is placed near to atoothed wheel 60 having teeth 62 with a pitch equal to T. The secondarywindings have an external diameter L which is greater than T/2, theresult of which is that the secondary windings cannot be aligned alongthe direction of displacement 64 of the teeth and for a maximumdifferential signal, but are aligned along a direction 66 making anangle α with the direction 64, such that tan a α=½.

The twelve secondary windings should be grouped two by two, i.e. incouples, in such a manner that the flanks that they detect will be ofopposite senses, i.e. one winding should be in correspondence with atransition in the sense of tooth-to-gap while the other is incorrespondence with a transition in the sense of gap-to-tooth.

Furthermore, the differential signals provided by the couples ofsecondary windings which correspond to the same type of transition areadded to augment the signal. Thus the signals of the couples (ES₁, ES₃),(ES₅, ES₇) and (ES₉, ES₁₁) are added by suitable wiring to obtain a sumsignal between the output terminals S₂ and S₁, while the signals of thecouples (ES₂, ES₄), (ES₆, ES₈) and (ES_(10,) ES₁₂) are added byappropriate wiring to obtain a sum signal between the output terminalsS₄ and S₃.

In FIG. 3 the terminals EP₁, and EP₂ are the input terminals of theprimary winding 14 and the arrows 90 indicate the sense of eachsecondary winding. In the device which is described with reference toFIGS. 2 and 3, the interconnection circuit 28 has been implementeddirectly on the substrate in the immediate vicinity of the secondarywindings ES₁ to ES₁₂ as a function of the pitch T of the teeth of thewheel 60, the diameter L of the secondary windings and the number ofsecondary windings. This corresponds to adaptation “by hand” inaccordance with the problem to be resolved.

The device which is described with reference to FIG. 4 corresponds to an“automatic” adaptation of the sensor to a differential configuration andto the pitch of the target. In this FIG. 4 there is representedschematically only the receiving part disposed in front of a singletooth 62 followed by a gap 68, the tooth and the gap being separated bya flank 70 which forms a line of magnetic discontinuity.

The secondary windings of a differential couple are located side by sideand the couples are denominated in the form of a matrix, each couplethus being referenced C_(ij). The 4 couples of the second row with j=2are staggered with reference to those of the first row with j=1. Theoutput terminals of the couples C_(ij) are connected to theinterconnection matrix 28, whose output terminals are connected to theamplification-demodulation circuit 32.

The adaptive circuit 36 comprises a comparator 80 and two matrixmemories 82 and 84.

The interconnection matrix 28 comprises a series of pairs of switchesI_(ij,) one pair per couple C_(ij,) in such a manner as to connect acouple selectively to the corresponding amplifier-demodulator circuit.It also comprises other switches, referenced Q_(ij) and K_(ij,) whichare provided to effect the interconnection between the different couplesin such a manner as to group them according to the results of theauto-calibration.

The auto-calibration is effected by controlling the switches I_(ij) whenthe target is stationary, so as to measure the output signal of eachcouple C_(ij). When the amplitude of the signal is greater than acertain threshold, this signifies that the couple is in correspondencewith a flank 70 (couple C₃₁) and this is stored in the memory 82 as thebinary signal 1. All the other couples are outside a flank and thedetected signals are less than the threshold; they are stored as abinary 0 digit. A matrix of 1's and 0's is thus obtained which, for thecase of FIG. 4 is:

0100

0000

Such a matrix signifies that the couple C₃₁ is in correspondence withthe flank 70, the couples C₄₁, C₂₃ and C₄₂ are facing a tooth while thecouples C₁₁, C₂₁, C₁₂ and C₂₂ are facing a gap. It is thus deduced thatit is necessary to add the signals of the couples in front of a toothand to subtract the signals of the couples in front of a gap and this isrealized by the switches Q_(ij) and K_(ij,) whose opening or closing iscontrolled by the binary values of a matrix derived from the preceding,namely:

1100

1100

where p=1 means addition of the signals and p=0 means subtraction of thesignals.

The foregoing description shows that the steps of an auto-calibrationprocedure of an inductive sensor consist of:

(1) measuring the output signal of each couple of secondary windingsC_(ij).

(2) comparing the measured signal with a threshold and assigning it thebinary value 1 or 0, according to whether it is above or below the saidthreshold,

(3) assigning the binary value 1 or 0 to the elements of a first matrixrepresenting the layout of the couples of secondary windings C_(ij),

(4) transforming this first matrix into a second matrix representinggroupings of couples of secondary windings to be effected according tothe discontinuity to be detected, and

(5) deriving control signals for the switches of the interconnectionmeans on the basis of this second matrix.

What is claimed is:
 1. An inductive sensor for detecting movements andmagnetic images of at least one target, comprising a primary winding(14) fed with an alternating current supplied by an excitation device(20) and a large number of secondary windings (16) subject to themagnetic field created by the primary winding (14) and providingelectric signals at their output terminals representative of variationsin the magnetic field due to the presence of a metallic target withdiscontinuities, said electric signals being applied to anamplification-demodulation circuit (30) followed by a processing circuit(34) for the demodulated signals, wherein the secondary windings (16)are of minimum dimensions compatible with detection of the signalsinduced by the variations in the magnetic field, in such a manner as tobe able to associate a maximum number of secondary windings with aprimary winding between two discontinuities of the target, and theoutput terminals of the secondary windings are connected tointerconnection means (28) and to the amplification-demodulation circuitwhich effect adaptive wiring of said output terminals in such a manneras to connect the windings among themselves for at least one of: (1)obtaining an electric signal from at least one of all of the secondarywindings, a partial grouping and a grouping which is variable with time,said groupings being effected by means of said interconnection means,(2) grouping the receiving windings among themselves in accordance witha predetermined representation, said representation being related to atleast one of the shape, the structure and the number of said at leastone target, and (3) grouping the receiving coils among themselves inaccordance with an auto-reconfiguration procedure effected in accordancewith detection of at least one of movement and magnetic imaging of atleast one target.
 2. The inductive sensor according to claim 1, whereinsaid interconnection means (28) comprises interconnection means forgrouping the secondary windings among themselves in such a manner as toallow detection of discontinuities of the target to maximize one of atarget signal value and a mounting error correction value.
 3. Theinductive sensor according to claim 1, wherein the interconnection means(28) are metallic conductor elements.
 4. The inductive sensor accordingto claim 1, wherein the interconnection means (20) comprise switchescontrolled by electric signals (M_(ij), I_(ij), Q_(ij) and K_(ij)). 5.The inductive sensor according to claim 4, further comprising anadaptive circuit (36) which provides the control signals for theswitches (Mij, Iij, Qij and Kij) of the interconnection means (28), saidcontrol signals (p) being determined by a calibration procedure
 38. 6.An auto-calibration procedure for an inductive sensor according to claim5, wherein said secondary windings are couples of secondary windings(C_(ij)), the procedure comprising the steps of: measuring outputsignals of each couple of said secondary windings (C_(ij)), comparingthe measured signal with a threshold and assigning the measured signalthe binary value 1 or 0 according to whether it is above or below saidthreshold, applying the binary value 1 or 0 to the elements of a firstmatrix representing the layout of the couples of secondary windings(C_(ij)), transforming said first matrix into a second matrixrepresenting groupings of couples of secondary windings to be effectedin accordance with the discontinuity to be detected, and derivingcontrol signals for the switches of the interconnection means on thebasis of said second matrix.
 7. An inductive sensor according to claim1, wherein said interconnection means is one of an activeinterconnection means and a passive interconnection means.
 8. Aninductive sensor according to claim 1, wherein said groupings areeffected by means of said interconnection means depending on at leastone of results of calculation, effecting a desired transfer function andresults of a desired averaging function.